New process helps 3D

While 3D printing technology does allow complex metal parts to be produced efficiently, such items often deform when stressed and heated. That may soon no longer be the case, however, thanks to a new technique developed at MIT.

The problem with existing 3D-printed metal components lies in a phenomenon known as "creep," in which persistent mechanical stress and high heat cause metals to permanently deform. Creep is particularly likely to occur when the metal is made up of fine grains, which is the case with metal that has been 3D printed.

Led by Prof. Zachary Cordero, a team at MIT has developed a heat-treatment process that makes those grains larger, and thus less susceptible to creep. It's a variation on an existing technique known as directional recrystallization.

In lab tests, 3D-printed nickel-alloy rods were initially placed in a room-temperature water bath directly below an induction coil, then slowly drawn straight up through the coil at various speeds. Doing so heated part of each rod to temperatures ranging from 1,200 ºC to 1,245 ºC (2,192 ºF to 2,273 ºF), producing a steep thermal gradient within the metal, between the coil and the water.

That gradient in turn caused the metal's microscopic grains to transform into much larger "columnar" grains. As the word implies, the new grains took the form of columns, which were aligned with the axis of greatest stress within the metal.

In the case of the rods, it was found that the optimum effect occurred at a temperature of 1,235 ºC (2,255 ºF) and a draw speed of 2.5 mm per hour – the scientists are working on increasing that speed. Needless to say, other combinations would likely work better for other metals. In fact, depending on the intended usage of the 3D-printed part, the grain structure could be varied within a single item, by changing the temperature and the speed as it was being treated.

Plans now call for the technology to be tested on structures resembling the blades of gas turbines or jet engines, which must endure ongoing mechanical stress and high heat. If they do indeed prove to be less prone to creep, it could pave the way for better, more efficient designs.

"New blade and vane geometries will enable more energy-efficient land-based gas turbines, as well as, eventually, aeroengines," said Cordero. "This could from a baseline perspective lead to lower carbon dioxide emissions, just through improved efficiency of these devices."

A paper on the research was recently published in the journal Additive Manufacturing.

Source: MIT